This new project analyzes the molecular architecture of synapses in the mammalian brain with an initial focus on the postsynaptic density (PSD). Many molecular components of the PSD are known, but the PSD is so enormous (approximately 10 to the ninth Da) that new methods had to be developed to understand how its numerous molecular components are organized. A promising new approach begins by attaching PSDs to glass and then freezing and replicating them for electron microscopy. Manipulations prior to freeing allow identification and localization of particular components. These include: orientation of the PSDs on the glass by prior treatment of the glass with antibodies to one of their surface components; partial digestion of the PSDs on the glass with detergents; and labelling, after either of the above with antibodies marked by colloidal gold. Work to date clearly distinguishes the cleft side of the PSD, where NR-1 and PSD-95 are localized, from the cytoplasmic side where CaMKII is localized. Digestion of the cleft side reveals a lattice-like structure that forms the core of the PSD; this lattice labels heavily for PSD-95. The distributions of spectrin and actin with the PSD are currently being investigated, and the distribution of CaMKII is the subject of another project. It is now apparent that the spatial distributions of components of the PSD are an important aspect of PSD function.